The present invention relates to a smoke generator apparatus and method and especially to the generation of smoke in a high temperature flame environment using an electrical resistance heating tube smoke generator.
Artificial smoke from smoke generators cannot be used for flame environment unless a special high temperature resistant smoke agent fluid is used. Only a few synthetic oils, such as Fyrquel 220, which is a synthetic oil made by AKZO Chemical, Inc., has been found acceptable for this purpose by the Navy for fire fighting training applications and more recently by civilian fire fighting trainers. These type trainers utilize non-smoking propane gas flames that have a need for supplemental training smoke for realistic training conditions. Another use for high temperature resistance smoke is for periodic leak testing of the many boiler systems that utilize high pressure vessels for leak testing and piping where cool down for leak testing is cost prohibitive and can be avoided by using high temperature smoke. So far, smoke agent fluid, such as Fyrquel 220, is the only material that is considered acceptable for the applications due to its high temperature resistance properties and relatively low toxicity.
This smoke agent currently requires special expensive equipment and cannot be used with existing conventional low cost, low temperature smoke generators. The reasons are not only its high operating temperature (about 550.degree. C.), but also the tendency for the heated fluid to rapidly decompose with subsequent clogging when exposed to moisture or moist air at elevated temperatures above about 90.degree.-200.degree. C. Hence, it is necessary to avoid moisture contact with the heated smoke agent fluid in the smoke generator system during operation and cool-down (when the system is shutting down). The smoke agent fluid is particularly susceptible to moisture contamination during cool-down since the boiling chamber (or heating tube) experiences a negative pressure permitting moist air to enter the heater. One existing smoke generator avoids this by forcing superheated air through a nozzle and inject the smoke agent fluid into this high velocity air stream. This method avoids the clogging problem, but is expensive due to heated air as a high temperature heat source and the high (50 psi) air blower pressure. Also, it has a low heat efficiency since the heat energy is lost by the hot air medium existing out the nozzle with the heated vapors. This results in high temperatures at the orifice exit that would tend to interfere with the condensation required for effective smoke generation. It also results in a relatively large package size.
Another more recent method of smoke generation is to utilize an electrical resistance tube heater for a low cost, efficient smoke producing system. What make this system possible is a blower that cools the heater coil to temperatures below 90 to 200.degree. C. very quickly when the power to the heater coil is shut off. During shutdown, the remaining hot fluid in the coil is initially boiled off and the coil is temporarily partially empty and subject to "stagnant heating" causing "cooking" of the wetted heater surfaces that are hottest near the outlet orifice. Over time, this would cause an accumulation of residue and require maintenance cleaning, primarily due to limitations of thermal mass on effective cool-down rate.
The conventional "low temperature" smoke generators are not amenable to the use of high temperature smoke agent due to their lengthy warm-up and cool-down times which presents residence (the time duration of fluid in contact with hot surfaces) heating problems. Another type of smoke generator utilizes a thin wall tube as an electrical resistance heater and has been used successfully for low temperatures of under 290.degree. C. for non-flammable environments. One version of this type of smoke generator requires expensive temperature control considerations due to the relatively low heater mass and long tube length. Either a special fast responding temperature controller that senses resistance changes with temperature distribution along the tube length or a special heat conductive, electrically non-conductive coating is required which permits the use of a point source temperature sensor or thermostat. The coating, however, also increases the start-up heating time and shut-down cooling time. These two approaches preclude their use as a high temperature smoke generator due to the excessive high temperature residence time during transient start-up and/or shut-down and high temperature moisture contamination during cool-down.
A smoke generator suitable for low temperature smoke generation can be seen in my prior U.S. Pat. No. 4,818,843. This prior smoke generator utilizes a thin wall tube as an electrical resistance heater and utilizes a thin coating over the resistance heating tube which electrically isolates the coils of the coil electrical resistance heating tube while conducting heat through the coils from the hot to the cold end of the coiled electrical resistance heating tube. Other smoke generators can be seen in Applicant's U.S. Pat. Nos. 4,547,656 and 4,568,820.
The present method of smoke generation utilizes a combination of a pressure relief valve for transient start-up as the viscosity and pressure changes with time and selective sizing of a multi-size diameter heater tube to match the temperature gradient and change in viscosity along the length of the heater tube. This permits an extreme reduction in heater tube size and an order of magnitude reduction in thermal mass for extremely rapid cooling. This makes it possible to essentially avoid the stagnant heating problem during cool-down. It also provides a feasible method for a higher capacity smoke generator using 220 V power source).